In the photomechanical trades, a mask containing an image that is opaque to actinic radiation is used in preparing a printing plate of some kind, e.g., letterpress, lithographic, etc. The mask is used as a phototool in exposing a layer of photosensitive resist-forming material present on a metal or plastic plate or a photopolymer printing plate matrix. After the exposure the printing plate is formed by etching or liquid development depending on the system present. The image in the mask must be of the highest possible contrast, e.g., completely opaque black in the image areas, and completely transparent (free from fog) in the unexposed areas. Silver halide film known as "litho" film is used to prepare the mask. When a litho film is exposed through a halftone screen and developed, it contains an image comprised of dots. The dots correspond to the areas of the film under the transparent areas of the halftone screen and are comprised of exposed and developed material. In silver halide litho film, these dots may be reduced in size by a process referred to as "dot-etching", i.e., reducing the size of, or "etching" the halftone dots, thereby changing the tone values of the image. In silver halide films dot-etching is accomplished chemically by treating the films with a silver "solvent". Silver halide films are expensive and require special red light handling.
Bratt and Cohen in U.S. Pat. No. 4,173,673 claim a dot-etchable mask prepared by imagewise exposure of a photopolymerizable element which comprises a support bearing a photopolymerizable layer having an optical density in the actinic region of at least 3.0 and having a thickness no greater than 0.015 mm. A removable cover sheet or a soluble overcoat layer can be present on the photopolymerizable layer. After the exposure through a halftone screen, the polymerized dots have a hardened upper skin which rests on softer undervolume having a lesser degree of polymerization or hardening. The dots are reducible in size by undercutting the polymeric dots with a solvent for the softer undervolume and removing hardened polymer from the edges of the hardened upper skins by mechanical action on the image bearing surface of the mask. The dot-etching of photopolymerizable element as described above requires the proper balance of the imagewise exposure and the undercutting of the polymeric dots with a solvent and mechanical action on the image bearing surface of the mask.
It is therefore desirable to provide a process whereby a dot-etched photopolymerizable lithographic film is prepared which does not require solvent undercutting of the polymerized image or dots as well as the use of mechanical action on the image or dots.